1. Field of the Invention
The present invention relates to optical devices and a method for manufacturing the optical devices.
2. Description of the Related Art
Along with advances in information communication networking in recent years, technologies for transmitting signals through the medium of light are being developed to realize higher-speed transmission of signals. One of the well-known technologies for signal transmission by light is a variety of optical transmission systems using optical waveguides as transmission lines.
For example, patent-related Reference (1) in the following Related Art List discloses a technology whereby an organic polymer material, which will form a core, is filled in a pattern by using a stamper (forming die) where the pattern corresponding to an optical waveguide is provided in the recessed shape, and then the organic polymer material is hardened under the influence of external energy such that the optical waveguide is formed projecting from a substrate surface. FIGS. 1A to 1C illustrate a manufacturing process of an optical waveguide using a stamper. As illustrated in FIG. 1A, a lower clad 112 is formed on a substrate 120 and then core solution 114a, which will constitute a core, is applied thereon. Then, as illustrated in FIG. 1B, a stamper 122 formed with a groove or recess which corresponds in form to an optical waveguide, is pressed against the core solution 114a, and in this condition, the core solution 114a is hardened by irradiating it with ultraviolet rays. In this manner, a core 114 is formed as illustrated in FIG. 1C.
Patent-related Reference (2) in the following Related Art List discloses a technology whereby an optical waveguide at a bent portion thereof, which connects in a curve two points whose optical axes of incoming and outgoing light are not aligned with each other, is divided into a plurality of narrow optical waveguides by clad portions provided inside the core at the bent portion. This structure can reduce optical loss, which may otherwise result from light leakage from the optical waveguide at the bent portion.
3. Related Art List
    (1) Japanese Patent Application Laid-Open No. Hei02-131202.    (2) Japanese Patent Application Laid-Open No. Hei09-145943.
However, according to the method described in the above patent-related Reference (1), core solution 114a is left over between a lower clad 112 and a stamper 122 when the stamper 122 is pressed against the core solution 114a, as shown in FIG. 1B. And if the core solution 114a is hardened in such a condition, a residual core layer will be formed on the lower clad 112 as shown in FIG. 1C. Furthermore, if the residual core layer is thick, some of light, as it travels through the core 114, will leak therefrom into the residual core layer, thus causing an optical loss. And if the optical loss is large, then the function of the optical waveguide will degrade.
An optical waveguide may be so designed that the core is bent or branched so as to propagate or branch light in a desired direction or directions. In such a case, however, light may leak from the core into the clad at a curved or branched point. And the leak of light into the clad creates a problem of increased optical loss. Moreover, the light leaking into the clad may enter a light receiving element, thus causing a drop in communication quality. Further, the light that has leaked into the clad may return to the core to produce interference with optical signals coming through the core, thereby creating a problem of added noise.
The technology introduced in the above patent-related Reference (2) has an objective of reducing such leakage of light from the core. However, the structure achieved by the method disclosed in Reference (2) is such that the core is branched into a plurality of cores at the bend and the branched cores are again joined into a single core. At the meeting point, therefore, lights having passed through different optical path lengths are brought together, which may produce interference and other problems.